Featured on the Ultimaker blog! https://ultimaker.com/en/resources/20921-drooloop-flowers

These are really fun to design and print. Each print job is fun since there's a slight element of randomness. The randomness is because we are deliberately "printing in the air" to create the "drooloops" (droop+loop) that make these flowers look so amazing.

I have uploaded both openscad design file AND the gcode files for ten different flower shapes that you can download and print right away. I have used a naming system so you should be able to see images of the design that goes with the gcode. The files print 4 flowers at a time, and I even adjusted the fan control so you’ll get nice symmetrical flowers like the ones you see in my pictures. Just try to match the filament diameter you are using (+-0.05mm) to name of the gcode files. I have found the gcode files will work with a range of diameters, since the majority of the printing  is just drooloops.

*********UPDATE June 2016!************
I have added some new gCodes file for the UM2+ using different nozzle sizes and updated the openSCAD file to make creating flowers for different size nozzles easier.
Also I have found that bed leveling is VERY important to getting a good print so I uploaded by bed leveling files here, these make it very easy to get your bed perfectly level in minutes without screwing around with paper, read the info here: https://www.youmagine.com/designs/bed-level-on-the-fly-print-pattern
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For those who love to tinker and want to make their own unique flowers, I have also uploaded all my source files, detailed instructions, and a video tutorial on how I modify the gcode by hand to obtain a more symmetrical flower shape. (The gcode fan tweaking is optional- don’t feel you need to do that much work.)

These slicer settings worked with 3 different printers I tried, and I bet will work with any slicers/printer if you use the models I have uploaded, or any you make with the openSCAD file as long as you are careful the advanced section of the file. 
ALL UNITS millimeters (mm) 
layer_height = 0.2 
wall_thickness = 0.8 
solid_layer_thickness = 0.6 
fill_density = 0 
support = None 
platform_adhesion = None 
solid_top = False (extra NOTE about solid_top: This is probably not your default setting....I use this so it will be hollow on top....for the pipe stem cleaner. I did not print the top, so this setting is kinda important to duplicate my results) 
solid_bottom = True

Feel free to experiment and let everyone know if you find improvements or just interesting results. One person's "interesting results" may be another person's "that exactly what I was looking for!".

I was able to get these to print fine on the three different machine I have access to (Ultimaker1, MakerGear M2, and TypeAMachines Series 1), but I bet you can get these out of any machine that is out there.

I will now go into gory details below. I learned a lot during this process and would like to share a few simple rules of thumb I use for the results I am getting, but I think all results look cool.

******(feel free to ignore everything below this line)**** 
Notes...about what I am doing to get more reliable and symmetrical results.

NOTE ABOUT SLICER SETTINGS -(my 1st choice for slicing is CURA -it's free, slices better than anything I have used and works with everything as far as I can tell) SO for a good clean drooloop you want the petals to print with no in-fill (see pictures of the comb test print). I like a simple drooloop to be formed by the nozzle leaving the center and coming back in a straight line and extruding the whole time (no retractions, or crazy infill motions). Look at your nozzle paths before you print to check this. If your slicer does not let you do this, then get one of the gcode viewers out there. I like hudborg's GCode Analyzer/Visualizer - http://www.thingiverse.com/thing:35248 

You can control the nozzle path and get these results by choosing model dimensions and slicer settings that compliment each other. For example make the petal width equal to the wall_thickness, so even if you are asking for infill none will be generated. 
A quick way to dial in your slicer settings VS petal widths is to use a calibration comb where each tooth is a different width and they cover a range of widths. Get one here:
https://www.youmagine.com/designs/custom-calibration-comb-openscad

I also want a nice thick drooloop so I never print finer that 0.2mm layer height. If you want to use different settings look at the advance setting area when generating your model.

FOR MORE SYMMETRICAL FLOWERS - TURN FAN OFF ONCE THE PETALS START PRINTING. If you don't turn off the fan it will be okay - your flowers may have a wind-blown effect since the petals may drift in the direction of the fan (see "Makes" for windy flowers). If you want, you can turn off the fan by watching the printer and turning off the fan with your controller (if you have one), BUT my favorite method (since not all my printers have controllers, and I prefer to only constantly monitor my printers while the 1st layers are going down) is to quickly and simply modify the gcode file with a simple text editor. This way I can print a bunch and know it will always work. Using a basic text editor, open the gcode text file and make these changes:

1. Turn off the fans on the layer just before the petals start to print. I usually add a comment to the layer name like "mark turned off fans" and then add right below that: 
   M106 S0 
   M107 
   This is kinda over kill, but i do it like that anyway. You can get the layer number needed by simple math: 
   p_start_height/p_height=layer# petals start 
   OR by using the layer viewer.

2. Search DOWN through the rest of the print looking for "M106" and just "comment out" those lines by adding a semicolon to the start of them... 
   so when you see: 
   M106 S58 
   you make it say: 
   ;M106 S58 
   (Depending on your slicer, settings, model, etc..you may or may not find a lot of these fan speed commands. NOTE: Make sure the changes are in the "petal printing area", since a full-document search and replace may turn off the fan before you want it to.)

here's a video that will help make this more clear: